Most high-speed optical channel links in short-reach data communication networks employ MMF. Transceivers that support these channel links use Vertical Cavity Surface Emitting Laser (VCSEL) sources for data rates of 1 Gb/s and higher. To achieve link distances in excess of 200 meters, the design of MMF has been optimized for VSCEL transmission with a center wavelength of 850 nm. An MMF optimized for VCSEL transmission is called Laser-Optimized MMF, and is specified in TIA-492AAAC and TIA-492AAAD as OM3 (fibre type Ala.2) OM4 (fibre type Ala.3) fiber types respectively.
Due to the wave nature of light and the wave guiding properties of optical fiber, an optical signal traverses the fiber along discrete optical paths called modes. The optical power of the pulse is carried by the sum of the discrete modes. The difference in propagation delays between the fastest and slowest modes in the fiber determines the inter-modal dispersion or simply modal dispersion. MMF should ideally be optimized so that all modes arrive at the output of the fiber at the same time to minimize modal dispersion. This has traditionally been achieved by shaping or “grading” the refractive index profile of the fiber core according to the parabolic distribution defined by
                                          n            2                    ⁡                      (            r            )                          =                  {                                                                                          n                    1                    2                                    ⁡                                      [                                          1                      -                                              2                        ⁢                        Δ                        ⁢                                                                                                  ⁢                                                                              n                            ⁡                                                          (                                                              r                                a                                                            )                                                                                α                                                                                      ]                                                                                                r                  ≤                  a                                                                                                      n                  2                                                                              r                  >                  a                                                                                        (        1        )            
Where, a is the core diameter (50 μm), n1 is the refractive index at the core center, n2 is the refractive index of the cladding, α is a number close to 2, and
                              Δ          ⁢                                          ⁢          n                =                                                            n                1                2                            -                              n                2                2                                                    2              ⁢                              n                1                2                                              .                                    (        2        )            
The traditional refractive index profile described by Equation (1) assumes that all modes have substantially the same wavelength and is considered the traditional “ideal” profile that results in minimum modal dispersion. Modes that travel with larger angles (and consequently traverse longer distances) encounter a lower refractive index on average and travel faster. These are called high-order modes. Modes traveling with small angles (low-order modes) encounter a higher refractive index on average and travel slower.
Much attention has been focused on minimizing modal dispersion of Laser-Optimized MMF by optimizing the refractive index profile of the fiber. Modal dispersion is the temporal distortion of an optical signal due to differences in the various modes' propagation velocities. Conversely, with respect to MMF, comparatively little attention has been focused on reducing the effects of material dispersion. Material dispersion is the temporal distortion of an optical signal due to differences in the propagation velocities of the various spectral components that comprise the optical signal. More generally, chromatic dispersion is a combination of material dispersion and waveguide dispersion where the waveguide properties change with wavelength.
As a result, it is desirable to provide an improved method for manufacturing MMF which accounts for and compensates for not only modal dispersion, but also material dispersion.
Additionally, due to the radially dependent wavelength emission pattern of laser transmitters, fiber-coupled modes have a radial wavelength dependency that results in appreciable material dispersion. Consequently, the total dispersion of the MMF system depends not only on the modal dispersion and material dispersion within MMF but also on the interaction between MMF and the emitting spectrum of the laser transmitter (oftentimes a VCSEL), all of which is governed by a fiber-coupled spatial spectral distribution.
The fiber-coupled spatial spectral distribution is dependent on the emitting spectrum of a laser transmitter which generates light radiation which travels down the MMF, and the manner in which the VCSEL's generated light radiation is coupled into the MMF. With reference to FIGS. 15 and 16, a Transmitter Optical Sub-Assembly (TOSA) 120 is used to couple light emitted from a VCSEL 124 into a fiber connector ferrule 132 mated to a transceiver 112 which houses both the TOSA 120 and a Receiver Optical Sub-Assembly (ROSA) 130. The ROSA is used for light detection. With reference to FIGS. 15 and 16, most generally, a TOSA comprises the following components: 1) a packaged VCSEL 124; 2) a lens 126; 3) a precision receptacle 128 for receiving a removable fiber connector ferrule 132; 4) a TOSA housing 121; and 5) an electrical connection 123 to a transceiver PCB. The packaged VCSEL 124 is most often packaged in a hermetically sealed package to improve device reliability. The lens 126 may be integrated into the packaged VCSEL 124 or molded into the TOSA housing 121. An illustration of a transceiver showing the TOSA is provided in FIG. 15 and a cross-sectional schematic of a TOSA is provided in FIG. 16.
For illustrative purposes only, it may be considered that the components of the TOSA 120 must be carefully aligned to achieve acceptable performance. An example assembly process for TOSA 120 may be summarized by the following steps. First, secure the lens 126 to the TOSA housing 121. Typically this is done by a press-fit, epoxy (thermal or UV cure) or laser welding. Second, position the TOSA housing 121 with lens 126 over an electronically addressable VCSEL 124. Third, insert the fiber connector ferrule 132 into the TOSA housing 121. Fourth, turn the VCSEL 124 on. Fifth, align the VCSEL 124 with respect to the TOSA housing 121, including lens 126 and fiber connector ferrule 132, to achieve the desired fiber-coupled power. Typically a 3-axis alignment is performed (x, y, z). Optical alignment of the TOSA 120 is typically achieved by inserting a fiber connector ferrule 132 into the receiving end of the TOSA 120 and optimizing for the maximum optical power as a function of VCSEL 124-to-package placement. Sixth, secure the VCSEL 124 to the TOSA housing 121. Typically this is done with epoxy (thermal or UV cure) or laser welding. Finally, remove the fiber connector ferrule 132 from the completed TOSA 120.
The different transverse modes of multimode VCSELs have different emission angles; higher-order modes have larger emission angles. It is also known that higher-order VCSEL modes have shorter wavelengths. When coupled into multimode fiber, the spectrum of the higher-order fiber modes will have a reduced central wavelength, λc compared to lower-order fiber modes. The measurement procedure described in TIA-455-127-A may be used to measure the emitting spectrum and determine its central wavelength, λc.
With reference to FIG. 17, when the components of TOSA 120 are pre-selected and aligned with tolerances within 1 mm or less, higher-order VCSEL modes are coupled into the higher-order fiber modes located farther from the center of the fiber core, λc outer. Conversely, lower-order VCSEL modes, which also have a longer central wavelength, are expected to be coupled into the lower-order fiber modes located near the fiber core center, λc inner.
However, with reference to FIG. 18, if the components of TOSA 120 are not precise and/or are in poor alignment, due to debris or misalignment issues such as VCSEL offset within the TOSA package or lens 126 is offset within the TOSA housing 121, the expected proportional relationship between VCSEL modes and fiber modes may not be realized. In fact, the optical system comprising the TOSA 120 may be such that higher-order VCSEL modes are coupled into low-order fiber modes and vice versa. It is essential to recognize that although imprecise components and/or poor alignment may result in optical aberrations, the fiber-coupled power may still exceed the specification minimum.
Numerous conditions, including environmental ones, may result in such a situation. Some examples include: 1) Misplacement of the VCSEL within the TOSA package; 2) Debris in the optical path; 3) Lens defects (e.g. moderate radius of curvature, excessive radius of curvature); 4) Thermal expansion (or contraction) of the various components comprising the TOSA; 5) Debris inside the ferrule bore preventing complete insertion; 6) Excessive ferrule concentricity; 7) Excessive fiber concentricity; and 8) TOSA housing defects.
There exists a need to have a transceiver that produces a predetermined fiber-coupled spatial spectral distribution that results in a material or chromatic dispersion that can be readily compensated for with a single fiber design. As a result, it would be desirable to provide an improved method for manufacturing a TOSA that produces a controlled fiber-coupled optical spectral distribution.